Copolymers of trioxane and cyclic ethers (which term is intended to include cyclic formals) having at least two adjacent carbon atoms, e.g., ethylene oxide, 1,3-dioxolane or 1,4-butanediol formal, have a wide range of use in industry, particularly as engineering resins. The mechanical properties of these copolymers, e.g., the degree of toughness of articles molded from them, depend to a large extent upon the molecular weight of the copolymer--with property improvements generally being obtained as molecular weight increases. The desirable property of resistance to thermal decomposition is a function of the degree of randomness of the distribution of the cyclic ether comonomer in the copolymer--the more random the distribution the better the copolymer's heat resistance.
Acetal or oxymethylene copolymers, including those made from trioxane and cyclic ethers having at least two adjacent carbon atoms such as ethylene oxide, 1,3-dioxolane or 1,4-butanediol formal as the comonomer pair, are well known to the art. See, for example, Walling et al. U.S. Pat. No. 3,027,352; Weissermel, K., et al., Kunststoffe 54:410 (1964); Collins, G. L., et al., J. Polymer Sci. (Chem.) 19:1597 (1981).
These copolymers have recurring oxymethylene units, i.e., --CH.sub.2 O--, interspersed with oxy(higher)alkylene units, e.g., --CH.sub.2 CH.sub.2 O--, and can be prepared, for example, as described in the Walling patent supra and in French Patent No. 1,221,148, by copolymerizing trioxane, the source of the --CH.sub.2 O-- unit, with a cyclic ether having at least two adjacent carbon atoms, in the presence of an initiator or a catalyst such as a BF.sub.3 dialkyletherate. Interestingly, when 1,3-dioxolane and ethylene oxide are copolymerized, it has been observed that the disappearance of the latter monomer conforms to pseudo first-order kinetics, and that the induction period for copolymerization can be eliminated by the addition of formaldehyde at a level equal to the sum of the equilibrium concentration and the molar concentration of the ethylene oxide. Price, M. B., et al., J. Macromol. Sci. (Chem), A1(2):23l (1967).
It has been demonstrated that, during the copolymerization of trioxane with ethylene oxide in the presence of the polymerization initiator BF.sub.3 dibutyletherate, a Bronsted acid is formed by the reaction of traces of water with the initiator. This acid complexes with trioxane causing depolymerization of the latter to formaldehyde. Thus, the species reacting with the cyclic ether in an insertion reaction is actually formaldehyde. Collins, supra.
Trioxane can be produced industrially by cyclic trimerization of aqueous formaldehyde with acid with azeotropic distillation of the trioxane as it is formed, Price, supra, while formaldehyde can be prepared commercially by cracking of gaseous trioxane. This invention obviates, in whole or in certain embodiments in part, the need to make and then purify trioxane only to decompose it to formaldehyde during copolymerization with a cyclic ether having at least two adjacent carbon atoms.
Direct copolymerization of formaldehyde and cyclic ethers having at least two adjacent carbon atoms is known in the art. Ishii, T., et al., U.S. Pat. No. 3,803,094; Grishin, B. P., et al., U.S. Pat. No. 4,340,720; Sakurai, H., et al., U.S. Pat. Nos. 4,377,677 and 4,399,272.
Ishi et al. disclose a process for copolymerizing formaldehyde and a cyclic ether in an inert organic solvent in the presence of a catalyst composed of a mixture of a methyl chelate compound, e.g., tris(acetylacetone)cobalt, with a Lewis acid, e.g., BF.sub.3 diethyletherate. Two polymerization methods are disclosed, one a batch solution method wherein the catalyst is added to a solution of the reactants in organic solvent, the second a "blow method" wherein gaseous formaldehyde and 1,3-dioxolane are continuously, and at a linear rate, blown into a solution of the catalyst in an organic solvent (see, e.g., Example 25). Rate constants of thermal degradation (K.sub.222 values) of the copolymer, which provide an estimate of the degree of randomness of the insertion of the comonomers, were high during the early stages of the solution polymerization method, but low (e.g., 0.02%/min.) at the end (see Table II). K.sub.222 values for the below method were not provided.
Grishin et al. disclose a process for producing high molecular weight oxymethylene copolymers in which gaseous formaldehyde (generated from methanol) is fed into a solution of a cyclic ether and an ionic catalyst in an organic solvent. When the process is practiced in a continuous mode, the vapor phase (formed as the result of the exothermic heat of polymerization) is withdrawn from, then recycled to, the reaction zone continuously. In this mode, the dispersed solid copolymer is drawn off continuously, while the solvent and reactants are added in amounts equal to the amounts withdrawn. The rate of admission to the reaction vessel of the gaseous reactants is said to be linear (column 5, lines 41-48 and column 6, lines 23-30) and is selected so as to insure a predetermined unit load of formaldehyde relative to the volume of the liquid phase (column 5, lines 49-51). No information is provided regarding the extent of non-random polymerization.
The Sakurai et al. '677 patent discloses the copolymerization of gaseous formaldehyde, added at a linear rate, with propylene oxide in the presence of polyethylene glycol monobutyl ether as the inert organic solvent and SnCl.sub.4 as the catalyst (column 14, lines 44-45). The degree of randomness of the copolymer is not disclosed.
The Sakurai et al. '272 patent discloses a process for producing a polyacetal copolymer wherein formaldehyde is copolymerized with an alkylene oxide, a poly(alkylene oxide), a cyclic formal or a poly(cyclic formal) in the presence of a Lewis acid and an anionic polymerization catalyst. Gaseous formaldehyde is added at a linear rate (column 9, line 53; column 10, line 2; Example 1, lines 8-10; Examples 2-3, lines 34-37). The final product exhibited a K.sub.222 value of 0.02%/min (column 11, line 8) at the end stages of the reaction.
Collart et al. U.S. Pat. No. 4,691,000 discloses copolymers containing oxymethylene and 2-fluoromethoxyethylene repeating units wherein the --CH.sub.2 O-- units are derived from trioxane and the 2-fluoromethoxyethylene units are derived from a fluoromethyl derivative of a cyclic ether. The rate of addition of trioxane to the reaction mixture is linear and is said to be predetermined by preliminary tests based on the desired proportion of oxymethylene units in the copolymer and the reactivity of the selected comonomer (column 3, lines 54-67). The randomness of the distribution of the 2-fluoromethoxyethylene comonomer in the copolymer is not disclosed.
None of the aforementioned patents or publications, nor any other patent or publication known to the inventor, teach a process for the preparation of an acetal copolymer by the copolymerization of formaldehyde with a cyclic ether containing at least two adjacent carbon atoms in which the rate of addition of gaseous formaldehyde to the reaction mixture is non-linear.